Passive infrared intrusion detection systems are known for sensing the presence of an intruder in a protected space and for providing an output signal representative of intruder detection. Examples of passive infrared intrusion detection systems are shown in U.S. Pat. Nos. 3,036,219; 3,524,180; 3,631,434; 3,703,718; and 3,886,360. It is an object of the present invention to provide a system and a mirror assembly therefor especially suited to ceiling mounting to produce a field of view through which an intruder must pass when moving about the floor area of a protected region, through which an intruder must pass when moving between the ceiling and the floor of the protected area, and through which an intruder must pass when moving on the floor and directly below the mirror assembly.